MODIFICATIONS TO THE NRC BELL 205 AIRBORNE SIMULATOR SAFETY SYSTEM IN RESPONSE TO A RECENT INCIDENT

The Bell 205 Airborne Simulator is a full authority fly-by-wire (FBW) research aircraft. On May 24, 1996 the NRC Bell 205 Airborne Simulator underwent a 4-axis actuator hardover upon engagement of the FBW system, with nearly catastrophic results. The sound design inherent in the current Bell 205 safety system allowed the safety pilot to override the FBW system and prevented the loss of aircraft and crew. However, during the resulting violent recovery, significant damage was caused to portions of the aircraft transmission mounting, drive shaft and intake bell mouth. Investigation has revealed that the likely cause of the hardover was the occurrence of a system reset in the VME-based flight control computer, which in turn caused a digital-to-analogue converter to reset to -10 volts, driving the aircraft actuators to full extension. As a result of this incident, many modifications were made to the Airborne Simulator safety system to prevent the reoccurrence of such an incident.